Focused Groundwater Controlled Feedbacks into the Hyporheic Zone During Baseflow Recession

Groundwater surface water interaction in the hyporheic zone remains an important challenge for water resources management and ecosystem restoration. In heterogeneous stratified glacial sediments, reach‐scale environments contain an uneven distribution of focused groundwater flow occurring simultaneously with diffusely discharging groundwater. This results in a variation of stream‐aquifer interactions, where focused flow systems are able to temporally dominate exchange processes. The research presented here investigates the direct and indirect influences focused groundwater discharge exerts on the hyporheic zone during baseflow recession. Field results demonstrate that as diffuse sources of groundwater deplete during baseflow recession, focused groundwater discharge remains constant. During baseflow recession the hyporheic zone is unable to expand, while the high nitrate concentration from focused discharge changes the chemistry of the stream. The final result is a higher concentration of nitrate in the hyporheic zone as this altered surface water infiltrates into the subsurface. This indirect coupling of focused groundwater discharge and the hyporheic zone is unaccounted for in hyporheic studies at this time. Results indicate important implications for the potential reduction of agricultural degradation of water quality.     

Temporal Hyporheic Zone Response to Water Table Fluctuations

Expansion and contraction of the hyporheic zone due to temporal hydrologic changes between stream and riparian aquifer influence the biogeochemical cycling capacity of streams. Theoretical studies have quantified the control of groundwater discharge on the depth of the hyporheic zone; however, observations of temporal groundwater controls are limited. In this study, we develop the concept of groundwater‐dominated differential hyporheic zone expansion to explain the temporal control of groundwater discharge on the hyporheic zone in a third‐order stream reach flowing through glacially derived terrain typical of the Great Lakes region. We define groundwater‐dominated differential expansion of the hyporheic zone as: differing rates and magnitudes of hyporheic zone expansion in response to seasonal vs. storm‐related water table fluctuation. Specific conductance and vertical hydraulic gradient measurements were used to map changes in the hyporheic zone during seasonal water table decline and storm events. Planar and riffle beds were monitored in order to distinguish the cause of increasing hyporheic zone depth. Planar bed seasonal expansion of the hyporheic zone was of a greater magnitude and longer in duration (weeks to months) than storm event expansion (hours to days). In contrast, the hyporheic zone beneath the riffle bed exhibited minimal expansion in response to seasonal groundwater decline compared to storm related expansion. Results indicated that fluctuation in the riparian water table controlled seasonal expansion of the hyporheic zone along the planar bed. This groundwater induced hyporheic zone expansion could increase the potential for biogeochemical cycling and natural attenuation.     

Effects of Multiscale Anisotropy on Basin and Hyporheic Groundwater Flow

Various subsurface flow systems exhibit a combination of small‐scale to large‐scale anisotropy in hydraulic conductivity (K). The large‐scale anisotropy results from systematic trends (e.g., exponential decrease or increase) of K with depth. We present a general two‐dimensional solution for calculation of topography‐driven groundwater flow considering both small‐ and large‐scale anisotropy in K. This solution can be applied to diverse systems with arbitrary head distribution and geometry of the water table boundary, such as basin or hyporheic flow. In a special case, this solution reduces to the well‐known Tóth model of uniform isotropic basin. We introduce an integral measure of flushing intensity that quantifies flushing at different depths. Using this solution, we simulate heads and streamlines and provide analyses of flow structure in the flow domain, relevant to basin analyses or hyporheic flow. It is shown that interactions between small‐scale anisotropy and large‐scale anisotropy strongly control the flow structure. In the classic Tóth flow model, the flushing intensity curves exhibit quasi‐exponential decrease with depth. The new measure is capable of capturing subtle changes in the flow structure. Our study shows that both small‐ and large‐scale anisotropy characteristics have substantial effects that need to be integrated into analysis of topography‐driven flow.     

Estimation of Submarine Groundwater Discharge

Leading methods for the evaluation of submarine groundwater discharge are presented, and their possible application under different hydrogeological conditions is discussed.     

Impacts of Streambed Heterogeneity and Anisotropy on Residence Time of Hyporheic Zone

The hyporheic zone (HZ), which is the region beneath or alongside a streambed, plays an important role in the stream's ecology. The duration that a water molecule or a solute remains within the HZ, or residence time (RT), is one of the most common metrics used to evaluate the function of the HZ. The RT is greatly influenced by the streambed's hydraulic conductivity (K), which is intrinsically difficult to characterize due to its heterogeneity and anisotropy. Many laboratory and numerical studies of the HZ have simplified the streambed K to a constant, thus producing RT values that may differ from those gathered from the field. Some studies have considered the heterogeneity of the HZ, but very few have accounted for anisotropy or the natural K distributions typically found in real streambeds. This study developed numerical models in MODFLOW to examine the influence of heterogeneity and anisotropy, and that of the natural K distribution in a streambed, on the RT of the HZ. Heterogeneity and anisotropy were both found to shorten the mean and median RTs while increasing the range of the RTs. Moreover, heterogeneous K fields arranged in a more orderly pattern had longer RTs than those with random K distributions. These results could facilitate the design of streambed K values and distributions to achieve the desired RT during river restoration. They could also assist the translation of results from the more commonly considered homogeneous and/or isotropic conditions into heterogeneous and anisotropic field situations.     

Freeze Shoe Sampler for the Collection of Hyporheic Zone Sediments and Porewater

The Starr and Ingleton (1992) drive point piston sampler (DPPS) design was modified by fitting it with a Murphy and Herkelrath (1996) type sample‐freezing drive shoe (SFDS), which uses liquid carbon dioxide as a cryogen. Liquid carbon dioxide was used to freeze sediments in the lower 0.1 m of the core and the drive‐point piston sealed the core at the top preserving the reductive‐oxidation (redox) sensitive sediments from the atmosphere and maintaining natural stratigraphy. The use of nitrogen gas to provide positive pressure on the gas system blocked the ingress of water which froze on contact with the cryogen thus blocking the gas lines with ice. With this adaptation to the gas system cores could be collected at greater depths beneath the static water level. This tool was used to collect intact saturated sediment cores from the hyporheic zone of the tidally influenced Fraser River in Vancouver, British Columbia, Canada where steep geochemical and microbial gradients develop within the interface between discharging anaerobic groundwater and recharging aerobic river water. In total, 25 cores driven through a 1.5 m sampling interval were collected from the river bed with a mean core recovery of 75%. The ability to deploy this method from a fishing vessel makes the tool more cost effective than traditional marine‐based drilling operations which often use barges, tug boats, and drilling rigs.     

Simulation of Groundwater Level in Ephemeral Streams with an Improved Groundwater Hydraulics Model

As a component of arid ecosystems, groundwater plays an important role in plant growth; therefore, it is essential to use deterministic models to reconstruct the process of groundwater level change. Typically, the linearized solution of the one-dimensional (1-D) Boussinesq equation yields acceptable performance in simulating transient conditions over short recharge periods in ephemeral stream systems, but the ability of this solution to simulate multiyear changes in groundwater levels is limited. In this study, an improved groundwater hydraulics (GH-D2) model is built based on the groundwater hydraulics (GH) solution of the 1-D Boussinesq equation to simulate multiyear changes in the groundwater level in ephemeral stream systems. The model is validated in the lower reaches of the Tarim River to simulate groundwater level fluctuations within the scope of influence of the river (300, 500, 750, 1050 m) over a 16-year period (2000 to 2015). To evaluate the performance of the models, the bias, mean absolute error, root mean squared error, Nash-Sutcliffe efficiency (NSE), and coefficient of determination (R²) are calculated. The results show that the improved GH-D2 model, which considers ephemeral streamflow, unsteady flow theory and the delayed response effect of groundwater level changes, performs well in simulating multiyear changes in the groundwater level in the ephemeral stream system. The observed and simulated values of the groundwater level at different river distances are consistent, and the model provides a new basis for multiyear simulations of groundwater level fluctuations in ephemeral stream systems.     

A Flux Detection Probe to Quantify Dynamic Groundwater-Surface Water Exchange in the Hyporheic Zone

A new probe was designed to quantify groundwater-surface water exchange in the hyporheic zone under dynamic stage condition. Current methods focus on either vertical pore water velocity or Darcy flux measurements. Both parameters must be understood to evaluate residence time and mass flux of constituents. Furthermore, most instruments are not well suited for monitoring instantaneous velocity or flux under dynamic exchange conditions. For this reason, the flux detection probe (FDP) was designed that employs electrogeophysical measurements to estimate in situ sediment porosity, which can be used to convert pore water velocity to Darcy flux. Dynamic pore water velocity is obtained by monitoring fluid conductivity and temperature along the FDP probe. Pressure sensors deployed at the top and bottom of the probe provide the additional information necessary to estimate vertical permeability. This study focuses on the use of a geophysical method to estimate pore water velocity, porosity, and permeability within a controlled soil column where simulated river water displaces simulated groundwater. The difference between probe derived and theoretical pore water velocity using natural tracers such as electrical conductivity and temperature was −4.9 and 3.9% for downward flow and 1.1 and 12.8% for upward flow, respectively. The difference in porosity calculated from mass and volume packed in the soil column and probe measure porosity ranged between −3.2% and 1.5%. Also, the calculated hydraulic conductivity differed from probe derived values by −8.9%.     

Freeze Core Sampling to Validate Time‐Lapse Resistivity Monitoring of the Hyporheic Zone

A freeze core sampler was used to characterize hyporheic zone storage during a stream tracer test. The pore water from the frozen core showed tracer lingered in the hyporheic zone after the tracer had returned to background concentration in collocated well samples. These results confirmed evidence of lingering subsurface tracer seen in time‐lapse electrical resistivity tomographs. The pore water exhibited brine exclusion (ion concentrations in ice lower than source water) in a sediment matrix, despite the fast freezing time. Although freeze core sampling provided qualitative evidence of lingering tracer, it proved difficult to quantify tracer concentration because the amount of brine exclusion during freezing could not be accurately determined. Nonetheless, the additional evidence for lingering tracer supports using time‐lapse resistivity to detect regions of low fluid mobility within the hyporheic zone that can act as chemically reactive zones of importance in stream health.     

Hydrogeological Conditions of Submarine Groundwater Discharge in the Crimea

A structural hydrogeological model of the Crimea based on the assumption of the leading role of faults in groundwater distribution is discussed. Areas with intense submarine groundwater discharge are outlined. The existence of water streams related to the faults in the area of the Chernaya River, Mramornaya Gully, Varnautskaya Depression, Batiliman Bay, and Nikitskii Cape is substantiated. The sites for drilling hydrogeological wells are recommended.     

On Estimating Chemical Discharge into the World Ocean with Groundwater

A quantitative estimate is made for the first time for the role of some chemicals discharged with groundwater into seas and oceans in the formation of hydrochemical regime in the coastal zone. The estimate is based on the regional and global regularities in the formation and distribution of submarine groundwater discharge into seas and oceans. A concept is proposed regarding the existence of geochemical and biological barriers on the submarine groundwater–seawater interface, which cause significant transformation and losses of many chemical elements.     

Wetland-Scale Mapping of Preferential Fresh Groundwater Discharge to the Colorado River

Quantitative evaluation of groundwater/surface water exchange dynamics is universally challenging in large river systems, because existing methodology often does not yield spatially-distributed data and is difficult to apply in deeper water. Here we apply a combined near-surface geophysical and direct groundwater chemical toolkit to refine fresh groundwater discharge estimates to the Colorado River through a 4-km² wetland that borders the town of Moab, Utah, USA. Preliminary characterization of raw electromagnetic imaging (EMI) data, collected by kayak and by walking, was used to guide additional direct-contact electrical measurements and installation of new monitoring wells. Chemical data from the wells strongly supported the EMI spatial characterization of preferential fresh groundwater discharge embedded in natural brine groundwaters and weighted to the southern wetland section. Inversion of the EMI data revealed sub-meter scale detail regarding bulk electrical conductivity zonation across approximately 15.5 km of transects, collected in only 3 days. This electrical detail indicates processes such as salinization of the unsaturated zone and direct discharge through the Colorado River sediments and a tributary creek bed. Overall, the study contributed to a substantial reduction in fresh groundwater discharge estimates previously made using sparse existing well data and a simplified assumption of diffuse fresh groundwater discharge below the entire wetland. EMI will likely become a widely used tool in systems with natural electrical contrast as groundwater/surface water hydrogeologists continue to recognize the prevalence of preferential groundwater discharge processes.     

Marine debris collects within the North Pacific Subtropical Convergence Zone

Floating marine debris, particularly derelict fishing gear, is a hazard to fish, marine mammals, turtles, sea birds, coral reefs, and even human activities. To ameliorate the economic and environmental impact of marine debris, we need to efficiently locate and retrieve dangerous debris at sea. Guided by satellite-derived information, we made four flights north of Hawaii in March and April 2005. During these aerial surveys, we observed over 1800 individual pieces of debris, including 122 derelict fishing nets. The largest debris concentrations were found just north of the North Pacific Transition Zone Chlorophyll Front (TZCF) within the North Pacific Subtropical Convergence Zone (STCZ). Debris densities were significantly correlated with sea-surface temperature (SST), chlorophyll-a concentration (Chla), and the gradient of Chla. A Debris Estimated Likelihood Index (DELI) was developed to predict where high concentrations of debris would be most likely in the North Pacific during spring and early summer.     

小江断裂带地下流体记震能力分析

收集并整理了云南地区小江断裂带附近十口流体观测井对印度尼西亚苏门答腊4次8级以上地震的响应资料．并对各观测井的记震能力及其流体测项间的响应协调性进行了统计分析。分析结果显示：物理量观测项的记震能力强于化学量观测项,水位测项的记震能力强于水温测项：小江断裂带南端各观测点的记震能力强于中部及北端各观测点：水位的同震响应形态多表现为震时振荡,而水温则多表现为震时脉冲下降。最后分析了记震协调性较好的高大、弥勒和嵩明三井的记震特征。     

Detecting Small Groundwater Discharge Springs Using Handheld Thermal Infrared Imagery

Ground‐based handheld thermal infrared imagery was used for the detection of small‐scale groundwater springs at the northwestern beach of Spiekeroog Island (northwest Germany). The surveys and in situ measurements of electric conductivity were carried out from shortly before to shortly after low tide along the low water line. Several brackish groundwater discharge springs with a diameter of 1–2 cm were observed along the beach at a distance of 2–3 m above the low water line. The high fresh water portion in the discharging water derives from the fresh water lens in the center of the island. During cold weather, the springs were identified by a significantly increased temperature (3–5 °C higher) and a lower electric conductivity (<10 mS/cm) in contrast to the surrounding sea water (1–2 °C, >30 mS/cm). During warmer weather conditions, an inverse temperature contrast was observed. The measurements confirm the applicability of thermal imagery for the detection of small‐scale groundwater discharge locations as an extension to the established method of aerial thermal scans and prove the existence of submarine groundwater seeps in porous systems. A ground‐based handheld thermal infrared imagery survey enables a precise installation of sampling devices as, for example, seepage meters.     

Groundwater Recharge Rate and Zone Structure Estimation Using PSOLVER Algorithm

The quantification of groundwater recharge is an important but challenging task in groundwater flow modeling because recharge varies spatially and temporally. The goal of this study is to present an innovative methodology to estimate groundwater recharge rates and zone structures for regional groundwater flow models. Here, the unknown recharge field is partitioned into a number of zones using Voronoi Tessellation (VT). The identified zone structure with the recharge rates is associated through a simulation‐optimization model that couples MODFLOW‐2000 and the hybrid PSOLVER optimization algorithm. Applicability of this procedure is tested on a previously developed groundwater flow model of the Tahtal? Watershed. Successive zone structure solutions are obtained in an additive manner and penalty functions are used in the procedure to obtain realistic and plausible solutions. One of these functions constrains the optimization by forcing the sum of recharge rates for the grid cells that coincide with the Tahtal? Watershed area to be equal to the areal recharge rate determined in the previous modeling by a separate precipitation‐runoff model. As a result, a six‐zone structure is selected as the best zone structure that represents the areal recharge distribution. Comparison to results of a previous model for the same study area reveals that the proposed procedure significantly improves model performance with respect to calibration statistics. The proposed identification procedure can be thought of as an effective way to determine the recharge zone structure for groundwater flow models, in particular for situations where tangible information about groundwater recharge distribution does not exist.     

Tracking Groundwater Discharge to a Large River using Tracers and Geophysics

Few studies have investigated large reaches of rivers in which multiple sources of groundwater are responsible for maintaining baseflow. This paper builds upon previous work undertaken along the Fitzroy River, one of the largest perennial river systems in north‐western Australia. Synoptic regional‐scale sampling of both river water and groundwater for a suite of environmental tracers (⁴He, ⁸⁷Sr/⁸⁶Sr, ²²²Rn and major ions), and subsequent modeling of tracer behavior in the river, has enabled definition and quantification of groundwater input from at least three different sources. We show unambiguous evidence of both shallow “local” groundwater, possibly recharged to alluvial aquifers beneath the adjacent floodplain during recent high‐flow events, and old “regional” groundwater introduced via artesian flow from deep confined aquifers. We also invoke hyporheic exchange and either bank return flow or parafluvial flow to account for background ²²²Rn activities and anomalous chloride trends along river reaches where there is no evidence of the local or regional groundwater inputs. Vertical conductivity sections acquired through an airborne electromagnetic (AEM) survey provide insights to the architecture of the aquifers associated with these sources and general groundwater quality characteristics. These data indicate fresh groundwater from about 300 m below ground preferentially discharging to the river, at locations consistent with those inferred from tracer data. The results demonstrate how sampling rivers for multiple environmental tracers of different types—including stable and radioactive isotopes, dissolved gases and major ions—can significantly improve conceptualization of groundwater—surface water interaction processes, particularly when coupled with geophysical techniques in complex hydrogeological settings.     

Groundwater flow through a Miliolite limestone aquifer

Abstract

This paper describes a study of groundwater flow in a coastal Miliolite limestone aquifer in western India. An examination of field information suggested that the transmissivity of the aquifer varies significantly between high and low groundwater heads. Pumping tests indicate that this is due to the development of major fissures in the upper part of the aquifer. A regional groundwater model with varying transmissivities is used to represent the field behaviour. The model is also used to examine the effect of artificial recharge on the alleviation of saline intrusion problems in the coastal area.     

Airborne Thermal Remote Sensing for Estimation of Groundwater Discharge to a River

Traditional methods for studying surface water and groundwater interactions have usually been limited to point measurements, such as geochemical sampling and seepage measurement. A new methodology is presented for quantifying groundwater discharge to a river, by using river surface temperature data obtained from airborne thermal infrared remote sensing technology. The Hot Spot Analysis toolkit in ArcGIS was used to calculate the percentage of groundwater discharge to a river relative to the total flow of the river. This methodology was evaluated in the midstream of the Heihe River in the arid and semiarid northwest China. The results show that the percentage of groundwater discharge relative to the total streamflow was as high as 28%, which is in good agreement with the results from previous geochemical studies. The data analysis methodology used in this study is based on the assumption that the river water is fully mixed except in the areas of extremely low flow velocity, which could lead to underestimation of the amount of groundwater discharge. Despite this limitation, this remote sensing‐based approach provides an efficient means of quantifying the surface water and groundwater interactions on a regional scale.